A link of this type is described, for example, in the article "Systeme urbain de transmission sur fibre optique unimodale a 140 et 34 Mbit/s (FOT140/34)" (A local monomode optical fiber transmission system at 140 Mbit/s and at 34 Mbit/s (FOT140/34)), published by D. Betoule et al. in the journal "Commutation et transmission," No. 2, 1988. This article also gives some information about the receiver, which receiver includes, for opto-electrical conversion, an avalanche photodiode (PDA) whose gain is controlled by an automatic level-controlling loop. Thereafter, the signal is amplified linearly by means of a low noise transimpedance stage. More precisely, it is specified that depending on the application, the attenuation along the optical fiber link between the transmitter and the receiver may lie within the range 0 dB to 28 dB, with the corresponding dynamic range at the receiver being obtained, on commissioning, by choosing between two operating ranges.
In such a link, when the attenuation is 28 dB, the received signal is weak. This requires low noise amplification. Gain must also be relatively high for the purpose of subsequent processing of the signal. However the attenuation may be only 0 dB if the link is extremely short or if the transmitter of an transmission link is looped directly to the receiver of the associated reception link for test purposes. If no precautions are taken, the photodiode used is in danger of providing a signal lying outside the operating range of the preamplifier, thereby making the link unuseable.
In addition to the case dealt with in the above-mentioned publication, various other solutions have also been provided to this problem.
Thus, proposals have been made to use an element which attenuates the light signal received by the photodiode, this element being in the form of an optical attenuator such as that constituting the subject matter of French patent document FR-A-85 17 277, or more simply merely adding an extra length of optical fiber. Such solutions are expensive and require human intervention during commissioning which makes them unacceptable in some applications.
In the article mentioned above, it is the sensitivity of the photodiode which is reduced by acting on its bias current. Another illustration of this type of solution is to be found in PCT patent document WO-A-85/04997. The variation in dynamic range obtained by such means is insufficient for satisfying requirements. In addition, in the article in question, it is mentioned that a dynamic range is selected during commissioning which means that it is unsuitable for solving the problem posed by tests which are performed by looping the transmitter back to the receiver without human intervention then being necessary.
Proposals have also been made to act on the dynamic range of the preamplifier. One such solution is illustrated, for example, by the transimpedance amplifier type TIA 1500 provided by the American firm MSC, Microwave Semiconductor Corp. Here too, the amplifier is limited with respect to variations in dynamic range. In addition, an amplifier that is modified in this way is relatively expensive.
The object of the present invention is to improve the operating dynamic range of an optical receiver having a current-generating photodetector and a low impedance electrical amplification stage in such a manner as to satisfy, in particular, the needs mentioned above, with this being achieved solely by use of means which are simple, cheap, and do not require human intervention.
To do this, the present invention proposes a current limiter circuit which, when applied to such a receiver, is interposed between the photodiode and the said amplification stage, and serves to limit the current applied to said stage, thereby preventing the applied current from going outside the operating range of the stage when the light signal received by the photodiode is at a high level. Naturally, such a limiter circuit can also be used in any similar application.